In a discharge system called dielectric barrier discharge or silent discharge, an alternating voltage is applied to oppositely placed electrodes with the high-potential electrode covered with a dielectric, so as to cause a discharge. This type of discharge is used in a wide variety of industrial applications that utilize chemical reactions with plasmas, because the discharge does not change to an arc and the electron temperature is stably kept high.
A particularly typical application thereof is that to ozonizers or ozone generating apparatuses, and so the dielectric barrier discharge is sometimes called ozonizer discharge. Other apparatuses that utilize this type of discharge include flat-plate light sources, carbon dioxide gas lasers, plasma displays, and the like. In particular, the electric operating region of flat-plate light sources is the closest to that of ozonizers.
Such ozone generating apparatuses and laser oscillators require power-supply devices for plasma generation. An example of such a plasma-generation power-supply device is disclosed in FIG. 12 of Patent Document 1. The structure of this example includes a discharging load in which a dielectric is interposed between a pair of oppositely placed electrodes to form a gas region serving as a discharging space, and the gas in this discharging space is excited to generate a plasma. The structure also includes a parallel inductor connected in parallel with the discharging load to improve the power factor. Electric power is supplied from an alternating-current power-supply to the discharging load through a rectifier, an inverter, and a transformer
Its operation will be described next. A commercial alternating voltage from the input power-supply is converted to a direct voltage by the rectifier, and further converted to an alternating voltage of a given frequency by the inverter. It is then boosted by the transformer to a voltage that starts discharge, and the high voltage is thus applied to the discharging load. The applied high voltage causes a discharge in the discharging load and the discharge excites gas particles.
Now, seen from an electric standpoint, the discharging load in which a dielectric is interposed between the discharging electrodes, i.e., the load using the dielectric barrier discharge, serves as a capacitor, and it is known that the current is advanced in phase with respect to the voltage. Accordingly, the power factor, expressed as a ratio between apparent power and active power, is low, and applying energy to the discharging load requires application of more current than necessary.
Therefore, the elements forming the transformer and the inverter need specifications capable of withstanding such current value, which leads to large-sizing and increased costs of the power-supply device.
The parallel inductor is connected as a phase delay component in parallel with the discharging load, in order to compensate for the lead of the current phase with respect to the voltage in the discharging load, and the lead of the current phase in the discharging load and the delay of the current phase by the parallel inductor are set equal to each other so that the phases of current and voltage supplied from the power-supply device match each other, which allows efficient application of power to the discharging load with minimum current. When individual components are ideal ones, then the power factor is 100% and a condition called resonance occurs.
In this way, in the conventional plasma-generation power-supply, the parallel inductor is connected in parallel with the discharging load to improve the power factor, so that the power-supply device can be smaller-sized and lower-priced with smaller-capacity power-supply elements.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-35693 (FIG. 12).
The load of such an apparatus using dielectric barrier discharge is characterized in that the electrostatic capacity of the load dynamically varies depending on whether the load is discharging or not. This is closely related to the design of the circuit for driving it, and Patent Document 1 mentioned above achieves the circuit design by representing the electrostatic capacity of the load with a value between an electrostatic capacity in no discharging and an electrostatic capacity in discharging, or with a representative electrostatic capacity in operation.
However, the representative electrostatic capacity in the operating state varies depending on the waveform condition, and the power applied to the ozonizer chiefly depends on the peak value of the voltage waveform. The representative electrostatic capacity in the operating state therefore depends also on the power applied to the ozonizer. This means that the resonance condition between the discharging load and the circuitry varies when the applied power is varied.
Patent Document 1 describes a method in which the resonance condition is set within a certain range, specifically between the electrostatic capacity in the non-discharging state and the electrostatic capacity in the discharging state, mentioning reasons that the resonance varies with a variation of the load, that the operation becomes sensitive at the resonance point, etc. This is certainly an effective method, but is disadvantageous when the capacity of the power-supply is to be minimized by making the power factor as high as possible when maximum power is applied and the power dissipation of the power-supply becomes maximum.
Also, the load may vary or the circuit constant may somewhat deviate. There is no guarantee that the load can be driven most suitably in such cases.
Also, even if the electrostatic capacity of the discharging load is strictly constant, the resonance condition depends on the applied power and therefore adjusting the applied power inevitably varies the resonance condition, and the driving condition deviates from the most suitable state.
Furthermore, when the maximum rated power is being applied and the load is driven with a high power factor in the vicinity of the resonance point, decreasing the applied power makes the control of the power-supply or discharge unstable. No measures have conventionally been taken against this phenomenon, and it is not even clear for what physical reasons this phenomenon occurs.